A light-emitting diode (referred to hereinafter as LED) represents one of the most popular light-emitting devices today. In recent years, the luminous efficacy of LEDs, defined in lumens per Watt, has increased significantly. LEDs are popularly used in many electronic devices, particularly handheld devices due to the power efficiency. Generally, LEDs are driven by light source drivers configured to provide a constant current such that the light emitted by the LEDs are consistent in terms of brightness and color. LEDs are generally driven using pulse width modulation scheme where the current are turned off by a switch. When the switch is turned on, the current flowing through the switch would be required be substantially consistent. Switches are usually implemented using MOS transistors that require build in potential. Generally, a power supply to a light source driver is required to have a voltage supply higher than all the build in potentials of those in the light sources and the switches.
In addition, as with other electronic devices, any light source driver may be susceptible to channel length modulation that is caused by fluctuations or variations in power supply. For example, operating voltage across a light source driver may drop when all the switches are turned on as a sudden increase in current is drawn from the power supply. The channel length modulation may cause abrupt changes in brightness and color. From the application perspective where color and brightness uniformity are highly sought after, the channel length modulation cancellation may be a feature that a light source drivers should have.
Similar to the requirements in all other portable devices, light source drivers are required to have as low power consumption as possible. For this reason, it may be desirable to operate LEDs at low voltage. However, this may be a challenge because LEDs and controlling switches may have built-in potentials. There may be a limitation as to whether how low the operating voltage can be reduced. To make the matter worse, LEDs may be required to be turned on fast so as to transmit signals at tens or even hundreds of mega hertz.
What is needed is a driver that can be operated under low voltage with high speed. Such light source driver may be used to drive not only LEDs, but also other light sources.
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